1. Field of the Invention
The present invention relates to methods and apparatus for real time fluid analysis and, more particularly, to a gas analyzer capable of determining concentrations of constituent gasses of a mixture as well as providing a direct quantitative measure of uptake, consumption and production of various inspired and respired gasses in real time.
2. Description of the Related Art
The determination of the relative concentrations of gasses in a mixture has been the subject of numerous inventions and intensive research over the years. Particularly, when noxious, poisonous or otherwise hazardous gasses are present, knowledge of the amount of such gasses is important to alert personnel in the area of any potential danger. In medical and clinical settings, awareness of the concentrations of respired gasses is important in the determination of patient metabolic conditions, especially the relative and absolute amounts of oxygen and carbon dioxide which provide information on the metabolization of oxygen as well as respiratory functioning. Under operating room conditions, anesthesiologists must be careful in administering anesthesia gasses and do so as a function of metabolic rate, and also must be aware of the absolute amount of anesthetic being provided in order to prevent overdosing or underdosing which would cause a patient to be aware during an operation. Also, when several different potent anesthetics must be administered during a procedure, the net amounts of the anesthetics need to be monitored to prevent overdosing.
Multiple medical gas monitors (MMGMs) continuously sample and measure inspired and exhaled (including end-tidal) concentrations of respiratory gasses, including anesthetic gasses during and immediately following administration of anesthesia. These monitors are required since an overdose of anesthetic agent, and/or too little oxygen, can lead to brain damage and death, whereas too little agent results in insufficient anesthesia and subsequent awareness. The current development of these monitoring devices is described in the extensive anesthesia and biomedical engineering literature. Complete and specific information about the principles and applications of these devices is well reviewed in several texts (see, e.g., Lake, Clinical Monitoring, WB Saunders Co., pp. 479-498 (ch. 8), 1990, incorporated herein by reference in its entirety), manufacturer's and trade publications (see, e.g., ECRI, "Multiple Medical Gas Monitors, Respired/Anesthetic", August 1983, incorporated herein by reference in its entirety), and in extensive anesthesia literature describing this equipment and its principles, methods and techniques of operation.
Medical gas monitoring provides the clinician with information about the patient's physiologic status, verifies that the appropriate concentrations of delivered gases are administered, and warns of equipment failure or abnormalities in the gas delivery system. These monitors display inspired and exhaled gas concentrations and may sound alarms to alert clinical personnel when the concentration of oxygen (O.sub.2), carbon dioxide (CO.sub.2), nitrous oxide (N.sub.2 O), or anesthetic agent falls outside the desired set limits.
Most MMGMs utilize side-stream monitoring wherein gas samples are aspirated from the breathing circuit through long, narrow-diameter tubing lines. it water trap, desiccant and/or filter may be used to remove water vapor and condensation from the sample before the gas sample reaches the analysis chamber. Gas samples are aspirated into the monitor at either an adjustable or a fixed flow rate, typically from 50 to 250 ml/min. Lower rates minimize the amount of gas removed from the breathing circuit and, therefore, from the patient's tidal volume; however, lower sampling flow rates increase the response time and typically reduce the accuracy of conventional measurements. These gas monitors eliminate the exhaust gas through a scavenging system or return certain gas constituents to the patient's breathing circuit.
Currently used anesthetic gas monitors employ one or a combination of methods and techniques to determine concentrations of respiratory gasses, including: mass spectroscopy, Raman spectroscopy, infrared light spectroscopy, photoacoustic spectroscopy, piezoelectric resonance, polarography, electrochemical fuel cells, paramagnetic analysis, and magnetoacoustics. Each of these techniques suffers from one or more limitations, including: the high cost and complexity of the equipment, the inability to provide real time measurements, the ability to measure concentration of only certain types of gasses or a limited number of gasses, inaccurate measurements, and the need for frequent equipment calibration. Another major disadvantage of most conventional gas monitors is that they do not measure nitrogen (N.sub.2). Safety considerations require that the presence of nitrogen be detected, since nitrogen detection provides warning of air embolisms, as well as alerting to possible loss of integrity of the breathing circuit, as air (with N.sub.2) is introduced. These conventional techniques and their drawbacks are described in U.S. patent application Ser. No. 09/104,997 to Drzewiecki (the present inventor), filed Jun. 26, 1998, entitled "Method and Apparatus for Real Time Gas Analysis, incorporated herein by reference in its entirety.
The Drzewiecki patent application relates to a universal method and apparatus for determining, in real time, the individual concentrations of fluid constituents of any mixture of a predetermined number of fluids (e.g., gasses or liquids) using, in the preferred embodiment, fluidic sensors. More specifically, the fluid (e.g., gas) analyzer disclosed therein comprises a side-stream sampling system, wherein a sample of the gas to be analyzed is drawn off (e.g., using a vacuum pump) and passed through fluidic oscillators, capillaries and an orifice which provide pressure drops and frequencies from which the properties (density, viscosity and specific heat) of the gas mixture can be determined in accordance with well-known relationships. The concentrations of the constituents are then derived from the measured mixture properties.
In particular, N equations, which from first principles, relate the individual fluid concentrations to measured properties of the mixture, are solved for the N unknown individual concentrations of the fluids in the mixture. N-1 properties of the mixture are measured by N-1 sensors, which from cost considerations are preferably fluidic sensors, but may be any other technology devices, and N-1 of the N equations are formed from the determined properties. The Nth equation is the constitutive equation, which requires that the sum of the unknown concentrations of the N known constituents be equal to unity.
While the fluid analysis techniques disclosed in the Drzewiecki patent application overcome virtually all of the limitations of the aforementioned conventional techniques, in certain cases, it is advantageous not to withdraw and analyze a side-stream sample in the manner described therein. Such situations include the monitoring of neonates whose tidal volume flow is so small that it approaches the required minimum side-stream sample flow, and cases where it is desirous to not operate a vacuum pump (because of noise or power considerations).
Moreover, in certain implementations, it may be advantageous to avoid side-stream sampling in order to integrate the gas analyzer into a monitoring system in a low cost manner with a minimum of sensors and complexity. For example, a gas analyzer employing the principles disclosed in the Drzewiecki patent application would be useful in a system for non-invasively monitoring metabolic rates, cardiac output and/or pulmonary function, such as that described in U.S. patent application Ser. No. 09/488,763 by Calkins et al., entitled "Non-Invasive Cardiac Output and Pulmonary Function Monitoring Using Respired Gas Analysis Techniques And Physiological Modeling", filed Jan. 21, 2000, incorporated herein by reference in its entirety. In this context, it would be desirable to integrate in a cost effective manner the sensors and elements required to determine gas concentrations with those capable of measuring end-tidal flow rates and volume in order to provide a direct quantitative measure of uptake, consumption and production of various inspired and respired gasses on a real time basis to support breath-by-breath determination of cardiac output and pulmonary function.
Further, the sensors used to measure the properties of fluid mixtures in the preferred embodiments described in the Drzewiecki patent application rely primarily upon fluidic elements. While these fluidic elements provide numerous benefits, in general, the sensors used to measure the properties of a gas mixture are not required to include fluidic elements. Indeed, in certain implementations, other type of elements may yield an overall simpler or lower cost design. Additionally, as alluded to the Drzewiecki patent application, properties other than those described in detail therein may be used to assay a gas mixture and to extend the fluid analysis principles disclosed therein to mixtures of a greater number of gasses.